1. Field of the Invention
The present invention relates generally to environmental testing and, more particularly, to a new methodology and production line system for Environmental Stress Screening (ESS).
2. Description of the Prior Art
Manufacturers have observed that devices experience a high failure rate immediately after being placed in service, often referred to as “infant mortality,” and near the end of their service life, often referred to as “old age.” During a device's “mid-life” between infant mortality and old age, failure rates usually drop precipitously to a much lower level than those experienced during infant mortality and old age. Consequently, if before shipping a product a manufacturer can reliably and economically identify and eliminate particular sub-assemblies or parts in those devices that may suffer from infant mortality, the manufacturer can significantly improve the reliability of its products, while also significantly reducing its warranty expenses. Studies of devices which experience infant mortality have shown that most structures, subassemblies or components fail because of a latent defect introduced into the device at the design phase, or from components or processes used during its manufacture. Unfortunately, only after a device or component experiences stress in its normal operating environment does the latent defect reveal itself by the device's or component's failure.
Environmental Stress Screening (ESS) is a screening process applied to devices or components before they enter service to ensure they are free of defects. Old techniques employed in ESS involve subjecting the device or component to vibration while concurrently operating the device or component and recording its operation. More recently, advanced ESS processes involve continuously monitoring a device's, or component's operation while exposing it to multiple stresses, such as vibration, ultra-high rate temperature cycling, humidity, powering off and on, electrical voltage variation, load variation, etc.
Studies have shown that advanced ESS techniques can significantly increase the reliability of a product. However, these processes are both expensive and time consuming, thus reducing the product throughput, especially for large systems. There are two difficulties when applying ESS to a large system. First, the large system is very cumbersome to handle, and second, it is very difficult and expensive to achieve the ultra-high temperature ramp rates required for an effective ESS process. Because of these inherent drawbacks in current ESS equipment, most manufacturers do not include the ESS procedure in the manufacturing process. To eliminate these shortcomings, this invention has developed a new process concept and new equipment that will drastically reduce the time required for the ESS process, thus increasing the throughput and therefore significantly reducing the cost.
In a complex electronic system, most defects occur principally in structures, connectors and printed circuit boards with attached components. To identify potential structural and inter-connection problems of a system, the ESS process should be performed on the entire system, but only for one or two units. Because all of these problems are usually mechanical, once the design or manufacturing process is corrected, it will prevent them from happening again. Therefore, the majority of the remaining defects are associated with the PC boards or subassemblies. To simplify our explanation, only PC boards will be used as an example. The ESS process should be applied to as many of the PCBs as possible. As the size and weight of PC boards are much smaller and lighter than large hassis, handling of these PC boards will be facilitated and, ultra-high temperature ramp rates and uniform vibration will be much easier to achieve.
In summary, this new approach to the screening process is to perform ESS on one or two units of the entire system to find the structural and inter-connection problems, and then screen the PC boards to identify and remove the flawed units before they are assembled into the system. This approach will drastically increase the reliability of the system, but with much reduced cost and screening time.
Although the U.S. Pat. No. 5,650,569 obtained by this inventor can impart vibration in all three axes (X, Y, & Z) to a system or PC board, it is only good for screening one system or a few boards at a time and not as efficient for screening many systems or boards. Also, for large systems it is difficult to achieve high ramp rate temperature cycling. Therefore, we are submitting the new process and system for patent to alleviate these problems.